1. Field of the Invention
The present invention relates to a ring oscillator, and in particular to a ring oscillator that prevents a reduction in an oscillation frequency due to a power voltage drop or a temperature rise.
2. Description of the Related Background Art
Recently, as the use of portable terminals or mobile products that employ batteries as power sources has spread, low-voltage operations have been requested even for integrated circuits in which non-volatile memory is mounted. Further, integrated circuits in which non-volatile memory is mounted are also used for vehicles, and in this case, there have been numerous requests for circuits that can function properly under high temperatures. In response to such requests, a booster circuit that generates high voltage for writing/erasing from externally supplied power source voltage is ordinarily mounted in non-volatile memory, and ring oscillators are commonly provided-as booster circuit constituents.
In a normal ring oscillator, wherein an odd number of CMOS inverters are connected to form a ring, the frequency of oscillation is reduced when the operating voltage is low or the temperature is high, and as a result of this reduction in the oscillation frequency, deterioration occurs in the capability of the booster circuit to produce and supply an output current, i.e., the capability of a power source to supply a current for the erasing of data from non-volatile memory and for the writing of data to non-volatile memory. Therefore, when the power voltage is low and the temperature is high, the periods required to write data to a memory cell and to erase data therefrom are extended, until in the worst case, data writing and erase are completely disabled. Therefore, to prevent a reduction in the capability of the booster circuit to supply a current, a ring oscillator is required that can maintain the oscillation frequency even when the power source voltage is low or the temperature is high.
A conventional example developed in response to this requirement is a voltage-controlled oscillator disclosed in U.S. Pat. No. 5,331,295.
FIG. 1 is a generalized circuit diagram of this conventional voltage-controlled oscillator, and FIG. 2 is a detailed circuit diagram showing the structure of the conventional voltage-controlled oscillator, excluding an oscillation unit.
In this conventional example, as is shown in FIG. 1, a voltage-controlled oscillator 40 comprises: an oscillation unit 52, wherein current starved inverters 115, which serve as delay cells, are provided at multiple steps in a cascade connection; a first current source 42, which is not affected by temperature and power voltage fluctuations; a second current source 44, for supplying a variable current that varies in accordance with the temperature and the power source provided voltage; an attenuator 46, for setting a current level for each cell in the oscillation unit 52; a reference voltage source 48; an input voltage node 50, for receiving an externally supplied control voltage; and a current mirror 110, connected between the attenuator 46 and the oscillation unit 52.
FIG. 2 depicts the detailed structures of the first current source 42, the second current source 44, the attenuator 46 and the current mirror 110, all of which are shown in FIG. 1.
In the conventional circuit, the current flowing across the first current source 42 is the total of the current received from the second current source 44 and the attenuator 46. The current received from the second *current source 44 is changed,in response to the temperature and the power voltage, whereas the current received from the first current source 42 is not affected by the temperature and the power voltage. Thus, the change in the current flowing across the attenuator 46 is the opposite of that produced by a change in the temperature and in the power source.
The oscillation frequency produced by the oscillation unit 52 is changed in accordance with the fluctuation in the temperature and in the power voltage. However, for the current starved inverters 115 of the delay cells that constitute the oscillation unit 52, the current level is set in accordance with the current supplied by the attenuator 46. Since the current flowing across the attenuator 46 is one for which the variation is the opposite of the current change that causes the temperature and the power voltage fluctuations, the oscillation frequency produced by the oscillation unit 52 does not depend on the temperature and the power voltage changes, and is determined only in accordance with a control voltage supplied by an input voltage node 50. Therefore, a stable voltage-controlled oscillator can be implemented.
As is described above, since the second current source, which supplies a variable current that is changed in accordance with the temperature and the power source voltage, is provided for the ring oscillator that is used for the conventional voltage-controlled oscillator in FIG. 1, temperature and power voltage fluctuations can be compensated for, and the oscillation frequency can be stabilized.
However, as is shown in FIG. 2, since the structure of the conventional voltage-controlled oscillator for the first current source 42 and the second current source 32 is complicated, and the size of the required layout is large, the dimensions of the integrated circuit chip and the manufacturing costs are increased. In addition, since as the resistance element constituting the reference voltage source 48 an accurate resistor is required that provides high resistance, this further increases the size of the layout.
To resolve these problems, it is one objective of the present invention to provide a ring oscillator that can prevent a reduction in the oscillation frequency when the power voltage drops or when the temperature rises, and that requires only a small layout.
According to the present invention, a ring oscillator comprises:
a current control unit, including
a basic current source having a series circuit formed of a first P channel transistor, the drain and the gate of which are connected, and a first N channel transistor, the drain of which is connected to the drain of the first P channel transistor and which, to supply a sub-threshold current, is operated in a weak inversion state,
a current mirror circuit having a series circuit formed of a second P channel transistor, the gate of which is connected to the drain of the first P channel transistor, a second N channel transistor, the drain and the gate of which are connected to the drain of the second P channel transistor; and
a ring oscillation unit, including
a current starved inverter, having
a current control P channel transistor, the source of which is connected to the power source and the gate of which is connected to the drain of the first P channel transistor of the basic current source,
a signal transmission P channel transistor, the source of which is connected to the drain of the current control P channel transistor,
a signal transmission N channel transistor, the drain of which is connected to the drain of the signal transmission P channel transistor, and
a current control N channel transistor, the drain of which is connected to the source of the signal transmission N channel transistor, the gate of which is connected to the drain of the second N channel transistor of the current mirror circuit, and the source of which is grounded,
wherein a connection point of the gate of the signal transmission P channel transistor and the gate of the signal transmission N channel transistor serves as a signal input terminal, and a connection point of the drain of the signal transmission P channel transistor and the drain of the signal transmission N channel transistor serves as an output terminal for outputting an inverted signal; and
an in-phase signal transmitter for receiving an input signal from the output terminal of the current starved inverter, and for outputting, to the input terminal of the current starved inverter and to an external device, a signal having the same phase as the input signal.
The above and relevant objectives and features of the present invention will become apparent by referring to the subsequent description, which is based on the accompanying drawings, and other innovative matters represented by claims.